** **No one knows the value of a dive plan better than Greg Powell. Movie directors precisely envision each scene, relying on a stunt coordinator to choreograph underwater sequences down to the second. A diver of 30 years, Powell, 58, started in the family business at age 14. To date, he has worked with Daniel Radcliffe, Cate Blanchett and Roger Moore. His film credits include For Your Eyes Only, Elizabeth and, most taxing, Harry Potter and the Goblet of Fire, which required six weeks of working with a specially built half-million-gallon tank.

Q: What’s the appeal of the underwater scene?

A: People like to see the unusual, and underwater is unusual for most people. Plus, audiences love seeing stars doing their own stunts. That’s what they’re paying for. If there is a gap where the double fills in, they feel cheated.

Q: Care to share any underwater tricks you’ve learned?

A: With Goblet of Fire, we had underwater wind machines, so to speak, creating a constant current. You never want to see an actor with hair across his face. It’s got to be moving and drifting.

Q: What’s the hardest element of your job?

A: You have to run through the stunts enough with any actor so that he feels 100 percent comfortable. You can’t just tell him: “You’ll be fine. Don’t worry about it.” If anyone is worried or panicky, it shows on his face. Actors are trying to give a different performance down there.

Q: How has your perception of diving changed?

A: I’ve got a much greater appreciation for the details. You’re reminded that it’s not as easy as putting in the regulator and going. It’s more like flying a helicopter. Silly mistakes could cost you everything.

Q: What is your dream project?

A: Anything that hasn’t been done -before. I would love to do an -underwater fight scene with horses, but horses don’t swim too well. Or an underwater car chase on a freeway. Whatever ideas the directors come up with, I’m up for the challenge.